Urban Life Aquatic: A Discovery In Paris Reveals A Huge Ecosystem Hidden In Plain Sight

For 12 days in June 2015, something rather unusual took place in Paris. Under clear skies, a team of scientists armed with toothbrushes, methodically passed through each of the city’s 20 districts searching for signs of life.

You can find many things in a busy European metropolis, but an aquatic ecology expedition isn’t usually one of them. Nevertheless, researchers from the Biology of Aquatic Organisms and Ecosystems research unit (BOREA) (CNRS / MNHN / IRD / UPMC / University of Caen / Université des Antilles) in Paris and the Max Plank Institute for Terrestrial Microbiology were on to something. They had noticed that the gutter water in Paris not only had the characteristic green and brown shades reminiscent of microbial matts, but they had also observed bubbles in the water — not the kind you get from detergent runoff, but the tiny spheres of pure oxygen that are formed during photosynthesis.

From these little bubbles came the discovery of an enormous ecosystem hidden in plain sight.

Urban ecological studies are showing how tree canopies can have a substantial impact on street temperature, water evaporation, and even flood mitigation. Other research is revealing how our highly curated urban biomes – with all their introduced plant and animal species — cause dramatic declines in local biodiversity. Only around 25% of native plant species and just 8% of native bird species manage to adapt to cosmopolitan life. As urbanization expands around many existing cities, and dramatically increases in some extremely biologically diverse areas of the world, it’s important to understand why certain plant, bird and mammal species are quite successful in cities, and others struggle to survive in the face of habitat loss.

But this isn’t the whole story. As we go smaller, the urban biome gets a lot bigger.

Under the microscope

Microbes are critical to the function of any ecosystem, and cities are no exception. Yet the extent to which urbanization stresses and distorts microbial diversity isn’t fully understood, which is kind of a problem because the microbial health of the urban biome strongly influences ecological health at the larger scale, including the health of its human inhabitants.

And there are, of course, microbes that interact directly with humans, some affecting health for the better and some for worse. As humans gather densely into urban centres, so to do our microbes. A unique ‘biogeography’ of human-associated microbes spans every city, and because urban areas tend to be highly partitioned, this can change from building to building, and even room to room.

Paris might be smaller, but with 6,100 streets, the gutter network is substantial and pervades the majority of the Parisian urban biome. So during those dozen rain-free days in early June 2015, the research team set out to explore the Parisian life aquatic.

And the toothbrushes? Those were used for scrubbing the microbial mats that covered the gutter surfaces. If you happened to have been in Paris at the time and witnessed this, you now finally have an explanation.

Collection Sites

The samples were geotagged and quickly sent back to the lab where they underwent genetic analysis. The researchers identified 5,782 Operational Taxonomic Units (OTUs).

The discovery of nearly this many distinct OTUs indicates a significant amount of biodiversity in Parisian gutters. Remarkably, the majority of these were not found in the non-potable water sources derived from local rivers, which are usually used for street cleaning. This suggests that most gutters had established a unique ecosystem of their own. The researchers were able to classify most of the microbes and found that the majority were diatoms, photosynthetic single-celled algae.

Diatoms first showed up around 200 million years ago (and perhaps earlier), and currently they are a critical part of the global food web, and account for 20% to 25% of total global photosynthesis. Those brownish films you see in wet gutters are most likely mats of algal diatoms, quietly fixing CO2.

The researchers also found that the gutters were teeming with a large number of other algal species, as well as other unicellular organisms, like alveolates. They also found fungi, and even a freshwater sponge species and a species of mollusk. While many of these are benign and downright fascinating, some of the organisms discovered were a concern.

While most of the fungi they found have no interest bothering humans, one of the OTUs identified may correspond to a genus of fungi that does include a human pathogen: C. neoformans, which can cause infections in immunocompromised people. While this particular species wasn’t found in the gutter study, the discovery of a close relation suggests that regular monitoring of the gutter microbiome is a good idea. So too does the discovery of an OTU that points to the presence of the Hartmannellidaefamily of amoebae. Amoebae are no strangers to natural or urban environments, the problem is that Hartmannellidae amoebae can act as hosts for some human pathogenic bacteria, including Legionella pneumophila which causes Legionnaire's disease. The amoeba-bacteria tenancy arrangement has probably been going on for around a billion and a half years, give or take. Bacteria find a way to evade an amoeba’s defenses and gain the advantage of its protective covering while hiding out. It lends further weight to the idea that monitoring gutter biodiversity could serve as an early warning system for public health hazards.

However, as the researchers point out in their paper, the gutter microbiome also has an important upside, “Understanding Street gutter communities’ composition and dynamic is an exciting new topic that might concern most of the urban and peri-urban areas worldwide.”

They propose that the gutter microbiome is likely to be beneficial in the treatment of water run-off, by breaking down human waste and vehicle pollutants. Given its substantial surface area in every city, it could also play a significant role in the global carbon cycle.

With this new insight into the biodiversity and structure of an often hidden layer of the urban biome, expect to see more urban ecology expeditions on a street corner near you.